In recent years, bipolar transistors and integrated circuits hybridizing bipolar transistors have been utilized in various application fields. The bipolar transistors have superiority to field effect transistors in view of high speed performance and high voltage withstanding performance and their use has been extended, for example, for devices in communication use or storage systems. As an existent example of a bipolar transistor capable of attaining higher operation speed or higher voltage withstanding, a silicon-germanium hetero bipolar transistor (SiGe HBT) using the selective epitaxial technology as shown in FIGS. 3A and 3B has been known. FIG. 3B shows the distribution of an impurity concentration Nc (cm−3) for phosphorous (P), boron (B), and antimony (Sb) as impurities, and FIG. 3A shows the distribution of germanium (Ge) in a main portion of a transistor in existent example 1. Literature concerned with transistors of such structure includes, for example, JP-A No. 10-79394 (1988).
For the sake of easy understanding of a relationship between the distribution of the Ge composition (%) and the impurity concentration Nc, an identical scale is used for the abscissa in common with FIGS. 3A and 3B. In FIG. 3B, are shown an emitter E, a base B and a collector C, as well as boron (B), phosphorus (P), antimony (Sb) in an n-type high concentration buried impurity layer (n+BL), respectively. There are also shown a depletion layer DP, an emitter-base junction jEB and a collector-base junction jBC. Also in FIGS. 1, 7, 14, 16, 17, and 19 to be described later, the constitutional portions identical with those in FIG. 3 carry same reference numerals.
The impurity concentration of the low concentration collector layer (n−Si) is controlled depending on the application use of a transistor. That is, increase of the concentration is intended by ion implantation or the like in a transistor intended for high speed operation, and the concentration is kept low for a transistor where an importance is attached to a high voltage withstanding characteristic.
The distribution of germanium is designed so as to cover the base region, and a hetero interface comprising the junction of silicon and silicon-germanium is formed in the emitter-base junction jEB. In a hetero bipolar transistor (HBT), change of the forbidden band width at the hetero interface near the emitter-base junction restricts the hole current flowing from the base to the emitter to provide an effect of improving the current gain, etc. On the other hand, on the side of the collector, since the change of the forbidden band width may possibly hinder the operation of a transistor, the hetero interface is designed so as to be at a certain distance from the collector-base junction jBC. In a case where the hetero interface is present near the collector-base junction, there is a high possibility that the hetero interface is situated in the p-type base layer under the effect of diffusion of base impurities due to the heat treatment upon preparation of the transistor, etc.
In this case, all discontinuous amounts of the forbidden band width appear as a barrier in the conduction band to greatly hinder the electron conduction and bring about remarkable decrease in the current gain and deterioration for the high speed operation of the transistor. In a case of an npn-bipolar transistor using silicon-germanium, when the hetero interface is present sufficiently from the collector-base junction jBC to the collector side, since all discontinuous amounts of the forbidden band width appear on the side of the valence electron band, the foregoing problem does not occur at least in the low current operation.
FIGS. 7A and 7B are views of a transistor structure showing existent example 2. For avoiding generation of a barrier in a conduction band Ec. It has been known that the height for the energy barrier generating at once is decreased by gradually decreasing the germanium composition on the side of the collector toward the high concentration buried impurity layer (Sb) in a silicon substrate as shown in FIG. 7A. FIG. 7A shows a germanium composition and FIG. 7B shows a distribution of an impurity concentration Nc. Literature concerned with the transistor of structure as in existent example 2 include, for example, JP-A No. 7 (1995)-147287.
Further, as existent example 3, it has been known to increase the working current by inserting a delta doping layer with the impurity concentration being increased in the collector near the base-collector interface. In this case, as shown in FIG. 9, transistor operation at higher collector current is possible while keeping the current gain at a stable value. Literatures Literature concerned with the transistor of the structure described above include, for example, JP-A No. 2002-359249.